I'm working on a conlang that is basically French but historically heavily influenced by the Florentine language. I have a few questions about cui.

Among speakers who only or mostly speak something close to standard Italian (many people these days), how often are the following standard constructions used? Are there non-standard constructions that also get used instead of these? (I'm not talking about constructions with quale, which are usually even more formal...)

Ser wrote:Among speakers who only or mostly speak something close to standard Italian (many people these days), how often are the following standard constructions used?

I would say almost always

Are there non-standard constructions that also get used instead of these?

I can't think of any

(I'm not talking about constructions with quale, which are usually even more formal...)

I'm not sure they're more formal. I would say they're just less used nowadays. They don't really give me an idea of formality to me.

Is it possible among some of such speakers, for example, to say "il mondo al che appartengo" or "il ragazzo che la (sua) sorella si chiama Carmela"

If there's one thing I've learned is that it's always best to not generalize on the entire Italian-speaking population, nonetheless I've never ever heard someone use such a construction and I'd be extremely surprised if I heard a native speaker say something like that.

il libro di cui ti parlavo

It's also very common to say il libro che ti dicevo

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As a rule of thumb, I would say that preposition + cui sounds totally fine to me, whereas article + cui might sound a bit stilted in an informal setting therefore I'd rephrase the sentence in another way so I don't have to use it. E.g: